10 replies to this topic

[sjamshid]

Hi Guys,

 

I am trying to perform a simple mass balance on a single stage desalter. I have 60000 BPD of oil with 4000 ptb, and 5% wash water with 400 ptb. I know that water effluent is 6667 BPD and crude oil effluent contains 0.2% water.

 

I know it should be a pretty simple and straightfowrard calc, but cannot wrap my mind around it. Can anyone please give some guidance?

 

Thanks in advance,

J

[ankur2061]

sjamshid,

 

Refer the attached sketch. The various terms in he sketch are defined as follows:

 

A = inlet water flow rate in oil, BPD

E = mixing efficiency

K_a = salinity of inlet water in oil, lb/bbl

K_c = salinity of outlet water in oil, lb/bbl

K_y = dilution water salinity, lb/bbl

Y = dilution water flow rate

C = flow rate of residual water in the outlet oil, BPD

V = disposal water flow rate, BPD

K_v = disposal water salinity, lb/bbl

k = salinity, ppm

SG_w = water specific gravity, dimensionless

Q₀ = oil flow rate, BPD

X_a = inlet water cut, expressed as fraction

X_c = outlet water cut, expressed as fraction

Z = oil salt content, PTB

 

Now try solving the mass balance. This should be enough guidance to give you a start.

 

Good luck in performing the mass balance calculations.

 

Regards,

Ankur.

 

Attached Files

•  Single_Stage_Desalter.jpg   28.12KB   253 downloads

[kkala]

Probably some data is missing, as following can indicate. "BPD", "ptb" quantities are supposed at some "standard" temperature.

1. Crude in : 60000 BPD (blue figures are given quantities).

    Water in: 60000x5% = 3000 BPD

    Water effluent (brine): 6667 BPD

    Crude out : 60000+3000-6667 = 56333 BPD  (assumed same as "crude oil effluent").

2. Salt with crude in : 60x4000 = 240000 lb/d (ptb=pounds per thousand barrels).

     Salt with water in : 4x400 = 1600 lb/d

     Total salt in : 240000+1600 = 241600 lb/d

     Assumed salt content in brine out: 33000 ptb, figure missing (red figures arbitrarily supposed for an example).

     Salt with brine out : 6.667x33000 = 220011  lb/d

     Salt with crude out : 241600 - 220011 =  21589 lb/d (*)

     Extent of salt removal from crude: 1-21589/240000  = 0.91, or 91%

3.1 "Desalting of Crude oil" can be seen in Nelson, Petroleum Refinery Engineering, McGraw-Hill, 1958), pages 265-268.

3.2 One could measure another quantity (e.g. salt content of crude out) instead of salt content of brine.

3.3  (*) Water content of 0.2% in crude out would mean 56333x0.2%=112.7 BPD of water. Assuming all diluted salt is in water, its salt content would be ~190000 ptb, which is unrealistically high (expected around 33000 ptb). Or probably 0.2% water is too low. 
On the other hand usual salt removal efficiency is ~ 90% (per 3.1). If the data is experimental, new measurement with additional data has to be realized, to clarify the balance.

3.4 Salt content of 35636 ptb in brine would result in "salt with crude out" 241600-6.667x35636 = 4015 lb/d,  and salt content 4015/0.1127 = 35625 ~ 35636 ptb in the water of crude out. This could close the salt balance in theory, but salt removal efficiency from crude would be  (240000-4015)/240000  = 98.3 % , which seems too high.

3.5 Comments on all above text are welcomed. Desalter brine met in local refinery had salt content ~1.5 g/l = 526 ptb, differing to present picture.

Edited by kkala, 25 February 2013 - 06:35 PM.

[ankur2061]

sjamshid,

 

When doing desalter calculations the wash water (dilution water) is generally not an input. It is a calculated value based on the mixing effciency of the wash water. If it were so simple as an input = output and with convenient assumptions then anybody with basic understanding of mass or material balance could do it.

 

As per the sketch I have provided with the notations, the wash water qty thus becomes: 

 

Y = A*(Kc - Ka) / E*(Ky - Kc) 

 

where:

 

Y = wash water flow rate, BPD

A = oil stream flow rate, BPD

E = mixing efficiency as a fraction (input, generally between 0.7 to 0.8)

Ka = salt content of water in inlet crude oil, lb/bbl (input)

Kc = salt content of water in outlet crude oil, lb/bbl (the desired value or desired output))

Ky = salt content of the wash water, lb/bbl (input)

 

Once you calculate Y, the water balance of the desalter gets solved

 

The brine disposal flow rate can be easily calculated:

 

V = A + Y - C

 

Once you know the values of A, Y, C & V, the salt content of the disposal (brine) stream becomes simple as per the salt balance  provided below and you don't have to make any assumptions.

 

Kv = A*Ka + Y*Ky - C*Kc / V

 

where:

Kv = salt content of the brine (waste stream), lb/bbl

 

Desalter calculations are thus done with the wash or dilution water as an unknown entity and not as an input.

 

Regards,

Ankur.

[kkala]

Concerning post no 4 (by ankur2061), following notes could be helpful for those seeking clarification of the query.

1. The matter concerns mass balance, not desalter design considerations.

2. Wash water input (stream Y on the thumbnail of post no 2) is given as 5% of crude oil input (local refinery: 3-5% of volumetric flow of crude).

3. To clarify stand of post no 4, please apply it using the specific numbers given in post no 1. Specific issues will get more clear.

4. Comments on post no 3 (by kkala) have also to be specific, preferably on calculated numbers (wrong, right?), to help readers and reality.

Edited by kkala, 26 February 2013 - 04:56 AM.

[ankur2061]

Basis of assumption of 33000 ptb salt content in brine has not been explained in post# 3. kkala is requested to provide the same. Post# 4 provides the basis for the wash water or dilution water calculated quantity based on the mixing efficiency which is how a mass balance is done for a desalter.

 

Nowhere post# 4 has tried to design the desalter (design, if somebody has the concept of it, means sizing of the desalter vessel). Only the mass balance is provided based on sound logic and with no assumptions. The mass balance provided in post#4 is based on the following inputs:

 

1. Flowrate of incoming oil

2. Water content of incoming oil

3. Salt content of incoming oil

4. Salt content of wash water

5. Desired output of salt in outgoing oil

6. Water cut of outgoing oil from the desalter

7. Mixing effciency

 

The outputs of the mass balance are as follows:

 

1. Wash water flow rate

2. Brine solution flow rate

3. Salt concentration of waste brine solution 

 

No assumptions have been made as regards to the content of salt in the brine solution which post#3 has so conveniently made without providing any explanation. No further explanation is required that this is how a mass balance across desalter needs to be done.

[kkala]

Post no 6 (by ankur2061) is noted, but let the status be cooled before a brief response. Meanwhile it may worth while looking into post no 3 (by kkala). Views can be of course different, readers can judge and take the useful part of each.

Edited by kkala, 26 February 2013 - 05:51 AM.

[narendrasony]

Derivation of the formula provided by Ankur for calculating Wash water flow rate (BPD)

Use following mass balance equations   (Refer Post-2 by Ankur for Nomenclature):

 

Water (or brine) balance:       

                                               A + Y = C + V                                                         - (1)

Deslater inlet water:                A + E * Y = B                                                         - (2)

From (1) & (2):                       C + V = B + Y * (1-E)                                              - (3)

 

 

Salt balance:                         

                                              A * Ka + E * Y * Ky  =  Kb * B =  Kb *  (A + E*Y)     - (4)     

                       Note that         Kb = Kc                                                                -  (5) 

                      Therefore        A*Ka + E*Y*Ky  =   Kc *  (A + E*Y)                          - (6)

 

 Or  Y = A*(Ka - Kc) / E*(Kc - Ky)  as given by Ankur.

 

 

Regards

Narendra

[kkala]

Negative feedback on post no 7 today by ankur2061 indicates that the status has not been cooled. Reasons are not apparent to kkala, thinking that stand of post no 3 is clear and helping for clarification of the truth. Impartiality is needed for this.

Ankur2061 is kindly requested for public explanation.

Brief response on technical matters by kkala, as said in post no 7, will come later.

[ankur2061]

As requested basis for assumption of 33,000 ptb in brine has not been provided. As mentioned in post #6 no assumptions have been made whereas post #3 makes an assumption which cannot be substantiated or has any basis. Thus the negative feedback.

[kkala]

ankur2061, on 04 Mar 2013 - 17:46, said
As requested basis for assumption of 33,000 ptb in brine has not been provided. As mentioned in post #6 no assumptions have been made whereas post #3 makes an assumption which cannot be substantiated or has any basis. Thus the negative feedback.

Probably some data is missing, as following can indicate.

...............................................................................................................

Assumed salt content in brine out: 33000 ptb, figure missing (red figures arbitrarily supposed for an example).

................................................................................................................

3.2 One could measure another quantity (e.g. salt content of crude out) instead of salt content of brine.

..................................................................................................................

3.4 Salt content of 35636 ptb in brine would result in "salt with crude out" 241600-6.667x35636 = 4015 lb/d,  and salt content 4015/0.1127 = 35625 ~ 35636 ptb in the water of crude out. This could close the salt balance in theory, but salt removal efficiency from crude would be  (240000-4015)/240000  = 98.3 % , which seems too high.

Post no 6 (by ankur2061) is noted, but let the status be cooled before a brief response. Meanwhile it may worth while looking into post no 3 (by kkala). Views can be of course different, readers can judge and take the useful part of each.

 

Impartial reading of kkala’s posts, especially of the points above, gives evidence that argumentation by ankur2061 in post no 10 is groundless. Nevertheless this is now subject to judgment of independent reader.

Looking into specific responses needs patience, it is not so easy to trace the mass balance problem. Generic directions are of little help, arithmetic application of given data could really help, and post no 3 has tried for it. Fair criticism should have studied “the other view” beforehand, or at least asked for clarifications; and not been in hurry.
Arithmetic application requested from ankur2061 would help simplify variables and probably go to the “theoretical” solution of about 35636 ptb of salt in brine (after adopting Kb=Kc, by naredrasony). One such solution has been already addressed in post no 3, para 3.4, by kkala, expressing also skepticism. Comments on it, or on resulting high salt content in brine, would have helped the query, contrary to politics and negative feedback.

Edited by kkala, 04 March 2013 - 06:29 PM.